1. Field of the Invention
The invention relates to an expansion turbine for low-temperature applications.
2. The Prior Art
The rotor of a turbine is connected with a metallic shaft over a large metal contact surface area with high contact pressure. The shaft forms a heat bridge in which a large flow of heat is dissipated into the turbine stage from the chamber receiving the roller bearing. The flow of heat being dissipated causes the roller support to falter. The danger that the lubricant may no longer be capable of flowing due to cooling and thus lose its lubricating power, is particularly pronounced with roller bearings lubricated with grease or a minimum amount of oil. The loss of heat of the roller bearings during rotation often makes it impossible to safely maintain the lubricant in a liquid state. Therefore, in low-temperature applications, roller bearings supporting the shaft of the rotor of an expansion turbine that are lubricated with a minimum amount of oil or with grease present serious problems.
An object of the present invention is to provide an expansion turbine for low-temperature applications having adequate cold insulation for the roller bearing on the turbine side. Therefore, interface lubrication and thus the function of the roller bearing during operation are assured.
These and other objects are accomplished by providing an insulating bush made of ceramic engineering material arranged between the inner ring of the roller bearing and the shaft. The insulating bush thermally separates the inner ring of the bearing from the cold shaft. According to another embodiment of the invention, the insulating bush has a disk-shaped shoulder on the turbine side that seals the space of the roller bearing of the turbine chamber from a loss of lubricant and thermally insulates this space. The disk-shaped shoulder has two functions: it acts as a centrifugal ring that prevents lubricant from exiting the space of the roller bearing, and it forms an insulating body that reduces the dissipation of heat from the space of the roller bearing into the turbine stage. It is also possible to substitute a separate disk for the disk-like shoulder made of the ceramic material. This separate disk is arranged on the shaft and seals the space of the roller bearing of the turbine stage against loss of lubricant and thermally separates it from the turbine stage.
The ceramic engineering material used for producing the insulating bush has low thermal conductivity, whereby the linear coefficient of thermal expansion of the material deviates not much from the steel grades used for producing the shaft and the inner ring of the bearing. Because the coefficients of thermal expansion of the materials are similar, it is assured in low-temperature applications that the minor installed clearance required for the function of high-precision roller bearings is maintained. In addition, the roller bearings are not thermally distorted and the inner bearing rings do not become detached. The ceramic engineering material of the insulating bush may have a thermal conductivity of 1.9 to 2.5 W/(mK) and comprises zirconium oxide and contains additions such as, for example yttrium oxide for stabilizing its structure.